Veporic Unit Research Articles

Petrology and monazite dating of the Fe-rich gneisses from Kokava (Veporic Unit, Western Carpathians, Slovakia): Devonian sediments supplied from Gondwanan-sources metamorphosed in the Variscan times

*Corresponding author The Fe-rich gneisses from vicinity of Kokava nad Rimavicou (Central Slovakia) were studied by electron microprobe to resolve the origin of the iron ore bodies. Whole-rock chemistry of the ironstones shows the lack of CaO, MgO and partly elevated P 2 O 5 contents. This, together with the mineral assemblage of almandine + magnetite + grunerite + ilmenite + quartz + apatite + allanite + zircon + monazite suggests an origin by metamorphism of sedimentary protolith and disproves the older idea about the formation as a skarn. The Fe–Ti oxides thermometry suggests temperatures of 360–420 °C, and the average oxygen fugacity of ∆log fO 2 normalized to the FMαQ of +0.13. The reconstructed oxygen fugacity for temperatures 500–600 °C (amphibolite facies) yields an interval of c. +1 to +3 ∆log fO 2 (FMαQ). The presence of detrital zircons, and monazites dated in this study, reveals a participation of lithologies of the pan-African orogen. The source rocks of the Devonian ironstones sedimented probably as oolitic chamosite in lagoons. The dated detrital monazite cores (612–400 Ma) show an affinity to source rocks formed in northern peri-Gondwana due to rifting and opening of the Rheic Ocean and separation of Avalonia and Armorica microcontinents. The majority of monazite data from the rims of detrital grains and from unzoned metamorphic grains bear an evidence of Meso-Variscan metamorphism with ages clustering between 360 and 320 Ma (342 ± 4Ma – weighted mean ± 2σ). The younger population with elevated Th contents (18.0–20.5 wt. %) giving the ages of 310–240 Ma (279 ± 2 Ma – weighted mean) is connected with the collapse of Variscan orogen and/or the onset of the early Alpine continental rifting. The Alpine ages are very rare, but a few monazites and one Th–U–Si inclusion in quartz yielded Cretaceous ages (115–85 Ma).

Two-stage breakdown of monazite by post-magmatic and metamorphic fluids: An example from the Veporic orthogneiss, Western Carpathians, Slovakia

The initial to advanced stage of monazite breakdown was identified in a granitic orthogneiss from the pre-Alpine basement in the Veľký Zelený Potok Valley (the Veporic Unit, Western Carpathians, central Slovakia). Monazite-(Ce) formed during Variscan metamorphism of the original Cambrian to Ordovician granitic rock. Two younger, Permian post-magmatic hydrothermal, and Cretaceous metamorphic-hydrothermal events caused a breakdown of the monazite to secondary egg-shaped coronal structures (100 to 500μm in diameter) with concentric newly-formed mineral phases. Two principal breakdown stages and newly formed mineral assemblages are recognizable: (1) partial to complete replacement of primary monazite with an internal apatite+ThSiO4 (huttonite or thorite) zone and an external allanite-(Ce) to clinozoisite zone; (2) hydroxylbastnäsite-(Ce) partly replacing apatite+ThSiO4 and allanite to clinozoisite aggregates. The monazite breakdown was initiated by fluid sources differing in composition. Stage (1) originated due to post-magmatic hydrothermal fluids, whereas stage (2) indicates an input of younger, CO2-bearing metamorphic-hydrothermal fluids.

Alpine reworking of Ordovician protoliths in the Western Carpathians: Geochronological and geochemical data on the Muráñ Gneiss Complex, Slovakia

Magmatic protoliths of Ordovician age have been identified in the metamorphic rocks of the Muráñ Gneiss Complex, Veporic Unit (Central Western Carpathians). Vapor digestion single zircon U–Pb dating yields an intrusion age of 464 ± 35 Ma (upper intercept) for the granite protolith. A lower intercept age of 88 ± 40 Ma records amphibolite-facies metamorphic overprint in the Cretaceous, during the Alpine orogeny. Geochemical and isotopic data suggest crustal origin of the orthogneiss. ɛNd initial are between − 2.6 and − 5.0 and T DM Nd between 1.3 and 1.5 Ga (two-step approach). 87Sr / 86Sr initial ratios vary between 0.7247 and 0.7120, and a steep REE pattern further constrains the crustal affinity of these rocks. Associated amphibolite bodies have ɛNd initial values of 6.5, 87Sr / 86Sr initial ratio of 0.7017, and a flat REE pattern. They are interpreted as MORB derived metabasites. Whole-rock Pb isotope analyses define a linear array in a 206Pb / 204Pb vs. 207Pb / 204Pb diagram with an age of ca. 134 Ma, consistent with intense Alpine metamorphism and deformation. These basement rocks of the Central Western Carpathians are interpreted as Ordovician magmatic rocks intruded at an active margin of Gondwana. They represent the eastern prolongation of Cambro–Ordovician units of the European Variscides, which were part of the peri-Gondwana superterrane and accreted to Laurussia during the Variscan orogeny. Variscan metamorphic overprint is not recorded by the isotopic data of the Muráñ Gneiss Complex. Alpine metamorphism is the most dominant overprint.

Electron-microprobe dating of monazites from Western Carpathian basement granitoids: plutonic evidence for an important Permian rifting event subsequent to Variscan crustal anatexis

Accessory monazites from 35 granitoid samples from the Western Carpathian basement have been analysed with the electron microprobe in an attempt to broadly constrain their formation ages, on the basis of their Th, U and Pb contents. The sample set includes representative granite types from the Tatric, Veporic and Gemeric tectonic units. In most cases Lower Carboniferous (Variscan) ages have been obtained. However, a much younger mid-Permian age has been recorded for the specialised S-type granites of the Gemeric Unit, and several small A- and S-type granite bodies in the Veporic Unit and the southern Tatric Unit. This distinct Permian plutonic activity in the southern part of the Western Carpathians is an important, although previously little considered geological feature. It appears to be not related to the Variscan orogeny and is interpreted here to reflect the onset of the Alpine orogenic cycle, with magma generation in response to continental rifting. The voluminous Carboniferous granitoid bodies in the Tatric and Veporic units comprise S- and I-type variants which document crustal anatexis accompanying the collapse of a compressional Variscan orogen sector. The Variscan magmas were most likely produced through the remelting of a subducted Precambrian volcanic arc-type crust which included both igneous and sedimentary reworked volcanic-arc material. Although the 2σ errors of the applied dating method are quite large and typically ±10–20 Ma for single samples, it would appear from the data that the Variscan S-type granitoids (333–367 Ma) are systematically older than the Variscan I-type granitoids (308–345 Ma). This feature is interpreted in terms of a prograde temperature evolution in the deeper parts of the post-collisional Variscan crust. In accordance with recently published zircon ages, this study shows that the Western Carpathian basement must be viewed as a distinct "eastern" tectonomagmatic province in the Variscan collision zone, where the post-collisional crustal melting processes occurred ~20 Ma earlier than in the central sector (South Bohemian Batholith, Hohe Tauern Batholith).

The Alpine overprints of the magnetic fabrics in the basement and cover rocks of the Veporic Unit (Western Carpathians, Slovakia)

In the Veporic Unit building up the central areas of the Western Carpathians, the magnetic fabrics are more or less coplanar and coaxial in metamorphic (Early Palaeozoic in age), granitic (Early to Late Palaeozoic in age) and sedimentary cover rocks (Permian/Triassic in age). The magnetic fabrics in metamorphic and granitoid basement rocks, as well as in sedimentary (-volcanic) cover rocks, appear to reflect a very wide P– T conditions of Cretaceous (originally Variscan) basement reactivation connected with a prograde metamorphism of the sedimentary cover rocks in lower greenschist (in the North-Veporic area) or the upper greenschist to lower amphibolite facies (in the South-Veporic area) during collisional burial in a collisional wedge. In addition, there are deformation zones where the magnetic fabrics formed due to a ductile deformation associated with extensional exhumation along (sinistral) lateral strike slip and normal faults. The late open folds did not principally change the former magnetic fabrics. The magnetic fabrics in sedimentary cover rocks of the North-Veporic area are either related to still recognizable bedding planes (in competent layers) with parallel superimposed metamorphic schistosity, or they are conformable to subparallel or oblique metamorphic cleavages. In the South-Veporic area, they are undoubtedly related to the metamorphic–mylonitic fabrics of the basement granitoids and their metamorphic mantle rocks. The magnetic fabrics in granitoid rocks of the South-Veporic area (S of the Pohorelá sinistral strike-slip fault) are influenced with micromechanisms of dominated low-temperature ductile deformation and partial dynamic recrystallization. We ascribe the observed coaxial magnetic fabrics in metamorphic, granitic and sedimentary rocks to an Alpine collision (110–95 Ma; P max∼8–9 kbar, T max∼530 °C) and extension tectonometamorphic events (95–80 Ma; up to P∼4–5 kbar, T∼300 °C) of Cretaceous age. The magnetic fabrics seem not to be strongly dependent on temperatures until the medium part of the amphibolite facies (as in this example), but they are strongly following simultaneous deformational low-temperature to medium-temperature meso- and microfabrics.

Obtaining of high quality talc from talcose rocks: a case study from the Sinec and Kokava deposits (Slovakia)

The talcose rocks of lower quality and rarely exploited rock varieties accompanying talc occurrences were tested aiming to obtain talc products of high quality. The main goal was to contribute to wasteless technology of processing, because these rocks are usually stored in dumps and cause environmental threat. The talc extraction and benefication from carbonatic talcose rocks - talc-magnesite, talc-dolomite and talc-magnesite-dolomite was done using flotation method in case study from Sinec deposit. The talc flakes, present in sericite-chlorite schistose rocks in Kokava deposit, were successfully tested for special use in paperboard for roof‘s covering. Both deposits belong to the strip of talcose occurrences in Veporic unit of Central Slovakia.

Comparative mineralogical and fluid inclusion study of the Hnúst'a-Mútnik talc-magnesite and Miková-Jedl'ovec magnesite deposit (Western Carpathians, Slovakia)

Two type localities, located in two different geological and metamorphic units, have been studied. The Hnúst'a-Mútnik deposit is located in amphibolite facies rock sequences of the Veporic unit, while the Miková-Jedl'ovec deposit (a part of the Dúbrava massive) is located in greenschists facies sequences of the Gemeric unit. On both localities successive crystallisation occurred during three stages of replacement. The first two belong to an older metamorphic process M1, the third corresponds to a younger M2 metamorphic process. During the first stage dolomite1 and calcite1 has formed on the expense of protolithic limestone. The second (major) stage is characterized by crystallization of magnesite. The third stage is represented by dolomite2, talc, chlorite, pyrite. At Hnúst'a-Mútnik this stage is much better developed and is further accompanied by tremolite, phlogopite, clinozoisite, zoisite. Based on the carbonate geothermometry at Hnúst'a-Mútnik the first stage occurred at 280-400°C and the third stage at 490-540°C; at Miková-Jedl'ovec the first stage crystallized at 370-420°C. Fluid inclusion study in magnesite showed the presence of primary brine inclusions, with high concentrations of salts other than NaCl, probably highly evolved evaporated marine waters. Brines from Miková-Jedl'ovec are slightly less saline (23-24 wt% NaCl eq.) and homogenised at lower temperatures (195-248°C) than the brines from Hnúst'a-Mútnik (29-32 wt% NaCl eq., 299- 348°C). Brines are accompanied by CO 2 -rich inclusions with nearly identical parameters at both deposits (1-8 wt% NaCl eq., CO 2 density 0.53 to 0.69 g.cm -3 , Th 307 to 336°C). CO 2 fluids probably result from dissolution of carbonates and are coeval or younger than brines. At Miková-Jedl'ovec also low salinity aqueous fluid inclusions have been identified (3-8 wt% NaCl eq., Th 132-249°C), corresponding to the stage 3. Microthermometric data have been also used to determine pressure and temperature limits related to the second and partially to the third stage of replacement.

Alpine metamorphic evolution and cooling history of the Veporic basement in northern Hungary: new petrological and geochronological constraints

Petrological and geochronological investigations were carried out on metamorphic rocks of the Veporic unit (Inner Western Carpathians) in northern Hungary. K/Ar and Ar/Ar data on micas and amphibole show only Alpine ages (mostly in the range of 87–95 Ma) in this basement unit. Thermobarometric calculations yield lower amphibolite facies peak conditions (ca. 550±30 °C and 9±1 kbar) for the Eoalpine metamorphic event. Complex evolution of gneissic rocks is reflected by the presence of discontinuously zoned garnets, the cores of which may represent relics of a pre-Alpine (presumably Variscan) thermal event. Zircon fission track (FT) data in the narrow range of 75–77.5 Ma indicate that this portion of the Veporic unit was emplaced to shallow crustal levels already during the Senonian time. The relative minor difference between zircon FT and K/Ar or Ar/Ar ages suggests very rapid cooling during the Late Cretaceous, most probably related to the extensional unroofing of the Veporic core complex. The obtained cooling ages do not support previous models of Tertiary uplift and exhumation of the Veporic unit along the Hurbanovo-Diosjenő Line.

Cretaceous evolution of a metamorphic core complex, the Veporic unit, Western Carpathians (Slovakia): P–T conditions and in situ40Ar/39Ar UV laser probe dating of metapelites

AbstractAlpine metamorphism, related to the development of a metamorphic core complex during Cretaceous orogenic events, has been recognized in the Veporic unit, Western Carpathians (Slovakia). Three metamorphic zones have been distinguished in the metapelites: 1, chloritoid + chlorite + garnet; 2, garnet + staurolite + chlorite; 3, staurolite + biotite + kyanite. The isograds separating the metamorphic zones have been modelled by discontinuous reactions in the system K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH). The isograds are roughly parallel to the north‐east‐dipping foliation related to extensional updoming along low‐angle normal faults. Thermobarometric data document increasing P–T conditions from c. 500 °C and 7–8 kbar to c. 620 °C and 9–10 kbar, reflecting a coherent metamorphic field gradient from greenschist to middle amphibolite facies. 40Ar/39Ar data obtained by high spatial resolution in situ ultraviolet (UV) laser ablation of white micas from the rock slabs constrain the timing of cooling and exhumation in the Late Cretaceous. Mean dates are between 77 and 72 Ma; however, individual white mica grains record a range of apparent 40Ar/39Ar ages indicating that cooling below the blocking temperature for argon diffusion was not instantaneous. The reconstructed metamorphic P–T–t path is ‘clockwise’, reflecting post‐burial decompression and cooling during a single Alpine orogenic cycle. The presented data suggest that the Veporic unit evolved as a metamorphic core complex during the Cretaceous growth of the Western Carpathian orogenic wedge. Metamorphism was related to collisional crustal shortening and stacking, following closure of the Meliata Ocean. Exhumation was accomplished by synorogenic (orogen‐parallel) extension and unroofing in an overall compressive regime.

Kinematics and metamorphism of a cretaceous core complex: The veporic unit of the Western Carpathians

The Veporic basement and its Permian-Mesozoic cover experienced medium-pressure, collision-related metamorphism during the Cretaceous. Geothermobarometric calculations of Alpine mineral assemblages indicate peak conditions of 8–12 kbar and 550–600°C in the deepest-exposed basement, and up to 8 kbar and 450–500°C in the Permian metasediments. After having reached the metamorphic peak conditions (at around 110 Ma, 40Ar/39Ar on amphiboles), the thermally softened Veporic unit was exhumed probably due to the underplating of a buoyant Tatric-Fatric crust. Exhumation was triggered by extensional denudation of former upper-crustal thrust units, overlying the Veporic unit. Unroofing was accomplished due to orogenparallel, top-to-east extension along low-angle, ductile normal shear zones. The area collapsed and rapidly cooled at 90-80 Ma (40Ar/39Ar on micas). As revealed by the structural record, the doming and tectonic exhumation of the Veporic core occurred in an overall contractional regime and was followed by additional Late Cretaceous—Early Tertiary shortening events.